1. Field of the Invention
This invention generally relates to wireless local area network (LAN) communications and, more particularly, to a system and method for establishing and/or maintaining a hybrid coordinator (HC) for an IEEE 802.11 wireless LAN.
2. Description of the Related Art
As noted in “A Short Tutorial on Wireless LANs and IEEE 802.11 by Lough, Blankenship and Krizman (computer.org/students/looking/summer97/ieee802), in addition to increased mobility, wireless LANs offer increased flexibility. An “ad hoc” network can be brought up and torn down in a very short time as needed. In IEEE's standard for wireless LANs (IEEE 802.11), there are two different ways to configure a network: ad-hoc and infrastructure. In the ad-hoc network, computers are brought together to form a network “on the fly.” There is no structure to the network; there are no fixed points; and usually every node is able to communicate with every other node. Although it seems that order would be difficult to maintain in this type of network, algorithms such as the spokesman election algorithm (SEA) have been designed to “elect” one machine as the base station (master) of the network with the others being slaves. Another algorithm in ad-hoc network architectures uses a broadcast and flooding method to all other nodes to establish who's who. The second type of network structure used in wireless LANs is the infrastructure. This architecture uses fixed network access points with which mobile nodes can communicate. These network access points are sometime connected to landline to widen the LAN's capability by bridging wireless nodes to other wired nodes. If service areas overlap, handoffs can occur. This structure is very similar to the present day cellular networks around the world.
The IEEE 802.11 standard places specifications on the parameters of both the physical (PHY) and medium access control (MAC) layers of the network. The PHY layer, which actually handles the transmission of data between nodes, can use either direct sequence spread spectrum, frequency-hopping spread spectrum, or infrared (IR) pulse position nodulation. IEEE 802.11 makes provisions for data rates of either 1 Mbps or 2 Mbps, and calls for operation in the 2.4-2.4835 GHz frequency band (in the case of spread-spectrum transmission), which is an unlicensed band for industrial, scientific, and medical (ISM) applications, and 300-428,000 OHz for IR transmission.
The MAC layer is a set of protocols that is responsible for maintaining order in the use of a shared medium. The 802.11 standard specifies a carrier sense multiple access with collision avoidance (CSMA/CA) protocol. In this protocol, when a node receives a packet to be transmitted, it first listens to ensure no other node is transmitting. If the channel is clear, it then transmits the packet. Otherwise, it chooses a random “backoff factor” which determines the amount of time the node must wait until it is allowed to transmit its packet. During periods in which the channel is, clear, the transmitting node decrements its backoff counter. When the channel is busy it does not decrement its backoff counter. When the backoff counter reaches zero, the node transmits the packet. Since the probability that two nodes will choose the same backoff factor is small, collisions between packets are minimized. Collision detection, as is employed in Ethernet, cannot be used for the radio frequency transmissions of IEEE 802.11. The reason for this is that when a node is transmitting it cannot hear any other node in the system which may be transmitting, since its own signal will drown out any others arriving at the node.
Whenever a packet is to be transmitted, the transmitting node first sends out a short ready-to-send (RTS) packet containing information on the length of the packet. If the receiving node hears the RTS, it responds with a short clear-to-send (CTS) packet. After this exchange, the transmitting node sends its packet. When the packet is received successfully, as determined by a cyclic redundancy check (CRC), the receiving node transmits an acknowledgment (ACK) packet. This back-and-forth exchange is necessary to avoid the “hidden node” problem. In the hidden-node situation node A can communicate with node B, and node B can communicate with node C, however, node A cannot communicate node C. Thus, for instance, although node A may sense the channel to be clear, node C may in fact be transmitting to node B. The protocol described above alerts node A that node B is busy, and hence it must wait before transmitting its packet.
As noted in “Ultra-Wideband Technologies for Short or Medium-Range Wireless Communications” (www.intel.org/technology/itj/q22001/articles/art 4g), the most important functions of the MAC layer for a wireless network include controlling channel access, maintaining Quality of Service (QoS), and providing security. Wireless links have characteristics that differ from those of fixed links, such as high packet loss rate bursts of packet loss, packet reordering, and large packet delay and packet delay variation. Furthermore, the wireless link characteristics are not constant and may vary in time and place. The mobility of users poses additional requirements, as the end-to-end path may be changed when users change their point of attachment. Users expect to receive the same QoS after they have changed their point of attachment. This implies that the new end-to-end path should also support the existing QoS (i.e., a reservation on the new path may be required), and problems arise when the new path cannot support the required QoS. Security is obviously an important consideration in wireless networks because, unlike wired networks, the overlaps between networks cannot be controlled. In addition, unauthorized user can also eavesdrop on transmissions. Security is handled through a combination of different means at the MAC layer, and also may include physical layer properties of the network.
In the IEEE 802.11 TGe committee, there is an ongoing project to enhance the 802.11 MAC to provide for prioritized channel access and QoS. The basic channel access function of the 802.11 MAC is the Distributed Coordination Function (DCF), with an optional mode called the Point Coordination Function (PCF) built atop the DCF, which offers a centralized, polling-based communication between stations and a point coordinator. With the PCF, the point coordinator defines a Contention-Free Period (CFP) during which the stations are polled and a Contention Period (CP) during which the normal DFC channel access mechanism holds. A periodic beacon identifies the start of the CFP and the duration. At the current stage, different prioritized channel access mechanisms for an Enhanced DCF (EDCF) mode are being considered. The EDCF mode provides for treating the priorities of different packets (encoded according to 3-bit traffic category tags) by giving them statistically fair access to the medium, is means that packets from the same priority class contend for the medium on equal basis according to the 802.11 MAC rules. Packets from different priority classes contend on a weighted basis, where the higher priority packets get a higher probability of success for channel access. Thus, higher priority classes cannot, in principle, choke transfer of lower priority class traffic under lightly loaded conditions. In addition to the EDCF modes, a type of point coordination function called the Hybrid Coordination Function (HCF) is also being proposed. The HCF mechanism provides for contention-free and controlled-contention transfers during any part of the frame (i.e., CFP or CP) by allowing the Hybrid Coordinator (HC) to generate bursts of CFPs, as opposed to a monolithic CFP. Thus, the HC can essentially create a number of ‘mini-CFPs” within the CP, as needed to meet traffic specs. Using this means the HCF promises to provide a flexible scheme where, for example, traffic classes that require periodic transmission opportunities can be accommodated within the CP or the CFP. Traffic that is burstier in nature is handled through the prioritized EDCF mechanism during the CP. In addition, this concept of CFP bursts is expected to mitigate the inter-cell interference that is a problem with the centrally controlled PCF mode when the cells are overlapping in extent.
As noted in US patent application Ser. No. 20020071449 (Ho et al.), a station may transmit by contention-free communications started and controlled by a HC. The HC may be a component of an access point or it may be a separate entity on the network. However, the HC is conventionally considered to be part of the same BSS as the stations that it is controlling. Transmissions during contention-free communications are ensured to be free of collisions because only one station within a given BSS has access to the communications medium at a given time. During contention-free communications either the station containing the HC or the station polled by the HC can transmit at a given time. Once a station has been polled, it is given access to the medium for a specified amount of time and is free to transmit information to any destination for the specified duration.
Alternatively, the station may transmit by contention communications coordinated by the HC. In order to transmit by contention communications, the station must first determine if the medium is idle and its backoff timer must be zero. If either condition is not met, then the station cannot transmit. However, even if both conditions are met, collisions may still occur, since more than one station may have attempted to transmit at the same time. Transmissions by contention communications typically are afflicted with collisions that will require one or more retransmissions after an extended delay.
According to the IEEE 802.11 technical standard, both contention-free and contention communications are supported. Contention-free communications is supported during a contention-free period (CFP) while contention communications is supported during a contention period (CP). Unfortunately for implementing QoS transfers, the CFP is an option in the IEEE 802.11 technical standard. Even if the CFP is provided, it uses a different set of access rules and frame formats than the CP. As a result, there are many IEEE 802.11 compliant wireless local area networks (LANS) that do not offer a contention-free period. In such networks, network latencies are generally large and spectrum utilization efficiencies are generally poor due to contention and collision.
In order to provide “Guaranteed Services” (delivery of packets to meet objectives for rate, delay and jitter) a form of polling is required. Currently, in 802.11e, the only way for polling to be accomplished is through a hybrid coordinator. Such a function is traditionally located in a non-mobile access point that controls a wireless network (called a BSS or basic service set).
In order to realize polling for mobile devices some means of making the hybrid coordinator access point mobile must be realized. This is not only a case of making a device smaller, it has to do with realizing a mechanism for allowing a stream to be maintained (at the application layer) as entities in the network move.
It would be advantageous if a reliable means could be established for providing guaranteed QoS in a wireless LAN.
It would be advantageous if a station (STA) could maintain the same level of QoS as it moves between BSSs.
It would be advantageous if wireless LAN HC could be established and maintained to manage the QoS.
It would be advantageous if guaranteed QoS could be provided for battery operated and portable wireless LAN systems, even if no external infrastructure network is present.